Multi-cylinder, reciprocating piston vehicle engines for driving automotive vehicles produce hot flowing exhaust gas streams which are treated after they leave the exhaust manifold of the engine to oxidize unburned hydrocarbons and carbon monoxide to carbon dioxide and water, and to reduce mixtures of nitrogen oxides (NOx) to nitrogen and water before the gas is released from the tailpipe into the atmosphere.
Many spark-ignited, gasoline engines are operated with fuel and air additions to the engine cylinders varying closely about the stoichiometric air-to-fuel mass ratio of about 14.7/1. Exhaust after-treatment is then accomplished using an oxygen sensor and a three-way catalyst system which is managed to promote both oxidation and reduction reactions for yielding a cleaned exhaust. Diesel fueled, compression-ignition engines, and other lean-burn engines are generally operated at air-to-fuel mass ratios that are well above the stoichiometric ratio and, thus, charge an abundance of air into the combustion cylinders. The exhaust from such engine operations contains more oxygen and nitrogen oxides than traditional gasoline engine exhaust. Exhaust treatment from lean-burn engines often uses an upstream oxidation catalyst for unburned hydrocarbons and carbon monoxide, and for oxidation of some NO to NO2. After passage through the oxidation catalyst, a reductant material for nitrogen oxides, such as urea, is injected into and mixed with the hot exhaust gas. The gas is then passed into contact with a catalyst material selected for a reaction between reductant material and nitrogen oxides to form nitrogen and water for release from the exhaust passage. The reaction is called a “reduction” reaction because the oxygen content of the nitrogen compounds is reduced. This exhaust gas reduction practice is often called selective catalytic reduction (SCR) of NOx.
SCR-type exhaust after-treatment systems require NOx sensors that are inserted in the exhaust stream for use in managing the addition of the reductant material to the exhaust stream and other after-treatment practices. Alternatively, or in addition, the NOx sensors can be used as part of an on-board diagnostic (OBD) system monitoring system that can to indicate the overall functionality of the vehicle emission system. NOx sensors are often formed as small electrochemical cells that function, for example, by producing voltage or electrical current signals responsive to the amount of nitrogen oxide species flowing in the exhaust and over sensor surfaces. NOx sensor data may be also used in assessing whether catalysts for NOx reduction, or other exhaust after-treatment materials, are working properly.
There is a need for on-vehicle systems and practices for determining whether a NOx sensor in an exhaust stream is performing properly. When a NOx sensor used in a vehicle exhaust system is not functioning properly, it is often necessary for the fault to be promptly diagnosed and reported to a vehicle operator. This disclosure pertains to on-vehicle, computer-conducted and managed diagnostic methods to assess certain aspects of the performance capabilities of these important NOx sensors.